High speed data transmission and communications are conventionally accomplished by transmitting communications carrier signals, such as optical or radio frequency ("RF") signals, from an optical or RF transmitter or one or more intermediate relay stations to a signal detector, such as an optical or RF detector, at the front end of a communications receiver. The communications carrier signals are typically formatted according to a predetermined communications standard which assigns the signal characteristics which define a logic "0" and a logic "1". One well known standard is the "Manchester" coding technique which assigns a logic "1" to a negative going signal transition and a logic "0" to a positive going signal transition.
Regardless of the signal carrier frequency employed or the communications standard used, the communications signals must be properly detected and decoded. Receivers in conventional high performance optical communications systems, for example, utilize optical detectors, such as avalanche photo diodes and other avalanche photo detectors (hereinafter collectively referred to as "APDs"), to detect low-level optical signals. In operation, an APD serves as a trigger or switch which generates an electrical output signal when exposed to an optical input. An APD converts the optical signal to a corresponding electrical signal and is thus well-suited to serve as the front end of an optical communications receiver.
APDs, other optical detectors and other signal detectors may produce signal-dependent multiplicative noise. When such a signal detector detects an optical, RF or other input signal, the electrical signal produced at the output of the detector includes multiplicative noise which is a function of input signal level. For APD detectors in optical communications systems, for example, such multiplicative noise includes shot noise which increases as the gain of the APD is increased. Thus, the shot noise generated by an APD which provides a relatively high gain may become a significant part of the resulting electrical signal. See, for example, S. M. Sze, Semiconductor Devices Physics and Technology, p. 286 (1985).
Signal-dependent multiplicative noise, such as shot noise in an optical communications system utilizing APDs for optical detection, may thus limit the communications data rate in the communications system by limiting transmitter power level and/or available receiver gain. More generally, the signal-dependent multiplicative noise results in a signal-to-noise reduction which may either reduce the data rate or increase the bit error rate (BER), since these two parameters may be traded off one for another.
In order to properly decode a Manchester encoded signal, i.e., to properly identify the data bit as a logic "1" or a logic "0", it is important to identify which phase segment of the resulting electrical signal has the positive-going transition and which phase segment has the negative-going transition. As the relative noise in the electrical signal increases, however, it becomes increasingly difficult to correctly identify the respective transitions in the phase segments and, as a result, increasingly difficult to decode the associated data bits.
Conventionally, communications systems have treated such multiplicative noise as if it were additive, averaging the noise level over the bit interval. As such, the overall sensitivity of such conventional communications systems has typically been degraded since the multiplicative noise that primarily occurs during only one of the phase segments is assigned an equal, albeit average, wieght during both phase segments, thereby corrupting both phase segments of the bit interval.
In order to detect relatively low level carrier input signals and enable high data rate communications, an APD or other signal detector must generally provide a relatively high gain, such as a gain of 200. However, conventional communication systems cannot increase detector gain without limit since the resulting electrical signals provided by the APD or other signal detector will include proportionally more noise. The multiplicative noise generated by an APD thus effectively limits the gain which can be provided by the detector without excessively increasing the BER.
A wide variety of signal detection techniques have been proposed. For example, U.S. Pat. No. 3,349,371 which issued to Brothman et al. and is entitled Quaternary Decision Logic proposes to distinguish between a binary "1" and a binary "0" by classifying the signal in one of four categories. This technique proposes to adjust system parameters, such as to increase or decrease the total transmitted energy, based on ambiguous signal recognition. However, Brothman et al. is relatively complex and can be computationally intensive since it measures noise over a long period of time. In addition, Brothman et al. does not address the resolution problems associated with the signal shot noise of an APD.
In addition, U.S. Pat. No. 5,175,507 which issued to Roither and is entitled Method Of and Device For Demodulating Biphase Modulated Signal describes a demodulation method which employs a reference phase angle level to determine and assign a binary value to a detected signal. However, Roither does not measure noise and also does not address resolution problems associated with increased noise levels.